In order to condition air that is to be supplied to an interior of a vehicle, vehicle air-conditioning units typically include a chiller for cooling the air and a glycol-air heat exchanger for heating the air, as well as glycol-air heat pumps and air-air heat pumps.
The current vehicle air-conditioning systems include various individual components. For example,                a condenser, arranged at a front of the vehicle;        a compressor, connected to and driven by an engine of the vehicle;        an air-conditioner, arranged in a passenger compartment to supply passengers with conditioned air; and        refrigerant lines.        
The above components are usually delivered individually to a vehicle production locale and mounted. Due to the number of components on a manufacturing line at a manufacturing plant, several assembly steps are required. In addition, a plurality of connections which have to be established during an assembly of the vehicle air-conditioning unit. As a result, potential leaks, which are costly to correct, may occur.
A solution of a premounted system is described in DE 102007046663 A1. An air conditioning system having a z-shaped arrangement of a blower-heat exchanger arrangement is disclosed. In addition, a closed, prefilled refrigeration loop is described in DE 102007046663
For a heating operation, vehicle air-conditioning units with air-glycol heat exchangers as glycol-air heat pumps are described in DE 102 53 357 A1 and JP7009844, and with air-air heat pumps in DE 42 44 137 C2, JP8216667 and JP2003291635, in the prior art.
Efficient refrigeration loops for the refrigerant R134a exist for a cooling operation.
Disadvantages of the prior art, which occur in the heating operation, are that, at low ambient temperatures, for example below −10° C., a cooling water temperature no longer reaches a level required for a comfortable heating of the passenger compartment in the case of efficient combustion engines, such as, diesel and Otto engines with direct injection. It is to be expected that these problems will worsen in future developments, such as, for example, in hybrid vehicles. For that reason, efficient auxiliary heating designs have to be used.
Further, there is a trend toward complete electrification of a drive system. Due to an efficient energy conversion from the drive battery waste heat available for heating the interior is further reduced. In addition, the energy quantity stored in the battery is considerably smaller than in the prior art. Thus, in future electric vehicles, power required for air conditioning the interior has a substantial influence on a range of the vehicle.
An additional disadvantage of the prior art with glycol-air heat pumps, is that the pumps use the cooling water of the combustion engine as a heat source. In the process, heat is removed from the cooling water. As a result, the combustion engine is operated for a longer duration at lower temperatures. This has a negative effect on exhaust gas emissions and on fuel consumption.
Due to an intermittent operation of the combustion engine in hybrid vehicles, a sufficient cooling water temperature is not reached, even in the case of longer trips. Consequently, a start-stop operation of the combustion engine is interrupted at low ambient temperatures.
Air-air heat pumps remove heat from the ambient air. Under some circumstances, this can lead to icing of the condenser, which functions as an evaporator when the heat pump is operated. If icing is avoided by way of an intelligent adjustment of the heat pump, the usable heating power of the heat pump is reduced as a result. If icing of the gas chiller is acceptable, the heat pump can be actively thawed by a brief operation of the refrigerant circuit as an air-conditioning unit. This reduces the average effective output of the heat pump.
Heat pump systems that release an output to the air are typically not capable of simultaneously dehumidifying and heating the air supplied to the vehicle. Consequently, the air conditioning of the vehicle, in case of low ambient temperatures, cannot be operated with recirculated air (i.e. air which has been recirculated from the vehicle interior.) Due to an absence of a dehumidification function, this would lead to inside steaming up of windows of the vehicle due to condensation.
Heat pump systems as well as fuel auxiliary heaters, which release their outputs to the engine cooling circuit, typically lack dynamics and present a low efficiency.
In DE 102009028522 A1, a compact air conditioning unit having a heat pump functionality is described. However, the heat pump functionality is implemented by an active switching of the cooling circuit. This leads to great complexity in the cooling circuit, which is associated with correspondingly high costs and technical risk.
In DE 602005004667 T2, an installation is described which works with absorbing plates. The system works cyclically, so that adjacently arranged heat-exchange units continuously switch from heating to cooling and vice versa. A second heat exchanger behaves anti-cyclically, and for that reason, for heating or cooling, a flap which can also be adjusted cyclically leads air, depending on the need, into a passenger compartment or into the environment. A reheating operation for cooling and dehumidifying the air, and subsequent reheating or continuous operation, are evidently not possible. Partial use of recirculated air is not described. Due to the cyclically moving flaps, the installation is technically very complex.
In DE 19824461 A1, a basic structure with heat exchangers arranged in parallel, and with parallel air flow, as well as discharge and mixing functions, is described. A reheating function or possibilities for partial use of recirculated air is not discernible. There is only one direction of flow where the air cannot be led out of the interior via the first heat exchanger, while outside air flows against the second heat exchanger.
In WO 2007042065 A1, a common holding fixture for components of an installation is described. A heating function, a partial use of recirculated air, or a mixing of the air flows is not discernible.
The problem of the invention now consists in providing an air conditioning system with heating functionality, which meets all the requirements, even in environments with heat sources of low capacity, such as, for example, in energy-efficient combustion engines or hybrid drives consisting of a combustion engine and an electric motor, or, in the case of absence of heat sources, of a drive system, such as, for example, in electrically driven motor vehicles; and in providing a method for the operation thereof.
It would be desirable to produce an air-conditioning unit with heat pump functionality which maximizes performance and efficiency and minimizes cost.